Adsorbent for metal ions and method of making and using

ABSTRACT

A method comprises the step of spray-drying a solution or slurry comprising (alkali metal or ammonium) (metal) hexacyanoferrate particles in a liquid, to provide monodisperse, substantially spherical particles in a yield of at least 70 percent of theoretical yield and having a particle size in the range of 1 to 500 micrometers, said particles being active towards Cs ions. The particles, which can be of a single salt or a combination of salts, can be used free flowing, in columns or beds, or entrapped in a nonwoven, fibrous web or matrix or a cast porous membrane, to selectively remove Cs ions from aqueous solutions.

The invention was made with Government support under Subcontract203690-A-F1, with Battelle Memorial Institute, Pacific NorthwestLaboratories, based on a Contract DE-AC06-76RLO-1830 awarded by theDepartment of Energy, and under Contract DE-AR21-96MC 33089, awarded byThe Department of Energy. The Government has certain rights in theinvention.

This is a divisional of application Ser. No. 08/918,113 filed Aug. 27,1997, now U.S. Pat. No. 5,935,380, which is a continuation-in-part ofapplication Ser. No. 08/612,528 filed Mar. 8, 1996, now abandoned.

FIELD OF THE INVENTION

The present invention pertains to substantially spherical, monodispersesorptive particles and a method therefor, and a method of using theparticles in loose form, in a column, or enmeshed in a web or membranefor extraction of cesium ions from solution. The method and particlesare useful in the remediation of nuclear wastes.

BACKGROUND OF THE INVENTION

Potassium cobalt hexacyanoferrate is known in the art as an effectiveadsorbent for cesium ions and has found important use in removingradioactive cesium ions from nuclear wastes via an ion exchange process.The method has been described, for example, in U.S. Pat. No. 3,296,123;in "A Review of Information on Ferrocyanate Solids for Removal of Cesiumfrom Solutions," P. A. Haas, Sep. Sci. Technol. 28 (17-18), 2479-2508(1993); and in "Evaluation of Selected Ion Exchangers for the Removal ofCesium and Strontium from MVST W-25 Supernate," J. L. Collins, et al.,ORNL/TM-12938, April 1995.

Potassium cobalt hexacyanoferrate, hereinafter referred to as "KCOHEX,"has typically been prepared by a method in granular form as described inU.S. Pat. No.3,296,123, wherein an aqueous acidic solution of potassiumferrocyanide is slowly mixed with an aqueous solution of cobalt nitrate,as shown in Formula I.

    potassium ferrocyanide+cobalt nitrate→KCOHEX+potassium nitrate

Water is removed by centrifugation, the wet cake is washed with water,then dried in an oven to form a dried solid mass. The solid mass isground and sized, and particles of from about 150 micrometers to about450 micrometers are packed into columns for subsequent exposure toradioactive wastes containing, in particular, Cesium-137.

The method of preparing particulate KCOHEX and other hexacyanoferratessuffers from two significant drawbacks. First, grinding the dried solidsmust be done carefully so as to minimize formation of unusable fines.Second, since a wide range of particle sizes results from grinding, theparticulate must be sized through sieves. These operations aretime-consuming and inevitably cause loss of product.

As described in the references noted above, sized KCOHEX is then loadedinto columns in order to remove cesium from radioactive waste solutions.

Spray-drying of solid materials is a method known in the art forpreparation of useful solids. See, for example, Kirk-Othmer Encyclopediaof Chemical Technology, 4th Ed., John Wiley & Sons, New York, 1993; Vol.8, p. 475-519, particularly pp. 505-508; and C. Strumillo and T. Kudra,"Drying: Principles, Applications and Design," Gordon and Breach, NewYork, 1986, pp. 352-359. However, in all of the reports of preparationand use of KCOHEX, a spray drying process has not been described.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a method comprising the step of:

spray-drying a solution or slurry comprising (alkali metal or ammonium)(metal) hexacyanoferrate particles, wherein metal is selected fromPeriodic Table (CAS version) Groups VIII, IB, and IIB to providemonodisperse, sorbent particles in a yield of at least 70 percent oftheoretical yield and having an average particle size in the range of 1to 500 micrometers, said sorbents being active towards Cs ions.Preferably, the hexacyanoferrate particles are substantially sphericalin shape and less than 20 percent by weight of particles have a size 5micrometers or smaller. Combinations of hexacyanoferrates can beespecially useful to remove Cs ions from aqueous solutions.

The sorbents preferably are selected from the group consisting of(potassium or ammonium) (metal) hexacyanoferrates wherein the metalpreferably is selected from Fe, Ru, Os, Rh, Ir, Co, Ni, Pd, Pt, Cu, Ag,Au, Zn, Cd, and Hg. More preferably, the sorbents are selected from thegroup consisting of potassium cobalt hexacyanoferrate (KCOHEX),potassium zinc hexacyanoferrate (KZNHEX), potassium ironhexacyanoferrate, potassium copper hexacyanoferrate, potassium nickelhexacyanoferrate, potassium cadmium hexacyanoferrate, and ammonium ironhexacyanoferrate. The liquid in the solution or slurry can be aqueous ororganic liquid.

In a further aspect, there are disclosed substantially spherical sorbentparticles having a particle size in the range of 1 to 500 micrometers,the particles being sorptive towards Cs ions in solution. Preferably,the spherical particles can be potassium cobalt hexacyanoferrate,potassium zinc hexacyanoferrate, potassium iron hexacyanoferrate,potassium copper hexacyanoferrate, potassium nickel hexacyanoferrate,potassium cadmium hexacyanoferrate, and ammonium iron hexacyanoferrate.The particles can be used in columns or beds to selectively remove Csions which can be radioactive from aqueous solutions.

In yet another aspect, the loose spray-dried sorbents which preferablyare potassium cobalt hexacyanoferrate, potassium zinc hexacyanoferrate,potassium nickel hexacyanoferrate, potassium copper hexacyanoferrate,potassium iron hexacyanoferrate, potassium cadmium hexacyanoferrate, orammonium iron hexacyanoferrate particles can be introduced into a Cs ioncontaining solution, equilibrated with the solution, and then separatedfrom the sorbed and/or exchanged metal ions.

In a still further aspect, the spherical sorptive particles that havebeen spray-dried can be enmeshed in nonwoven, fibrous webs, matrices, ormembranes. The webs, matrices, or membranes, which preferably areporous, can be used in solid phase extraction (SPE) procedures toselectively remove Cs ions from aqueous solutions.

In yet another aspect, the invention provides an SPE device, such as acartridge which in preferred embodiments can be pleated or spirallywound, comprising a fibrous non-woven SPE web comprising spherical,monodisperse particles which, in preferred embodiments, can be any ofpotassium cobalt hexacyanoferrate, potassium zinc hexacyanoferrate,potassium iron hexacyanoferrate, potassium copper hexacyanoferrate,potassium nickel hexacyanoferrate, potassium cadmium hexacyanoferrate,and ammonium iron hexacyanoferrate or potassium nickel hexacyanoferrateparticulate and aramid fibers enclosed in a cartridge device.Preferably, tile web is porous.

In yet another embodiment, the invention provides a method of removingthe specified metal ions from an aqueous solution comprising passing theaqueous solution by or through a fibrous non-woven SPE web or matrix ormembrane comprising the above-described spherical, monodisperseparticles which can be any of the sorptive particles made by the methodof this invention, the particles preferably being any ofhexacyanoferrates disclosed herein. Preferably, the web is porous.

In another embodiment, the invention provides a method of removing thespecified metal ion from an aqueous solution comprising passing theaqueous solution through an SPE column comprising spherical,monodisperse sorptive particles made by the method of this invention,the particles preferably being any of hexacyanoferrate particulatesdisclosed herein.

In yet another aspect, the method further comprises the step of heatingspherical metal or ammonium hexacyanoferrate particles, after drying, ata temperature of at least 115° C., preferably 115 to 130° C. for up to12 hours. In the case of KCOHEX, the particles can be heated until theircolor changes from green to purplish-black.

In this application,

"size" means the diameter of a spherical particle or the largestdimension of an irregularly shaped particle;

"monodisperse" means a monomodal particle size distribution (i.e.,particles of uniform size in a dispersed phase preferably having anaverage size range of 1 up to about 60 micrometers, preferably about 5to about 30 micrometers, as illustrated by FIG. 1;

"drain time" means the time required to dewater a slurry of particlesand fibers when making a sheet; and

"substantially spherical" means particles that are spherical, ovoid(having an elliptical cross-section), raspberry-like, or toroidal, andare free of sharp corners;

"particles" and "particulate" are used interchangeably;

"web", "matrix", and "membrane" are used interchangeably and each termincludes the others;

"sorptive" or "sorb" means by one or both of absorption and adsorption;and

"heavy metal" refers to metals having a molecular weight of at least 50;and

The overall process yield in making particles of the invention using aspray-dryer with a diameter of at least 1 meter is at least 70 percentpreferably 80-90 percent, or more compared to a yield of about 60percent or less when using prior art ground and sieved particles.Preferably, the resulting particles are free of submicron sizeparticles, with not more than 20 percent of particles being <5 μm insize. The spray-drying process substantially eliminates productparticles having submicron sizes.

Additionally, free-flowing spherical particles will pack with pointcontact in columns, resulting in less channeling and a lower pressuredrop during extraction compared with, for example, irregularly shapedprior art particles having the same average size. Irregularly shapedprior art particles, particularly those less than 5 micrometers in sizecan pack tightly and lead to a high pressure drop in extractionapplications. When irregularly shaped prior art particles are greaterthan 50 micrometers in size, channeling can result as liquids passthrough, resulting in poor separations.

Further, the advantages of the particles of the invention includereduced drain time by a factor of at least about 1.5 times or morecompared to non-spherical, irregularly shaped, prior art particlestypically obtained from a grinding process, when incorporated in a sheetarticle.

Further, sheet articles can be made using particles of this invention,whereas in many cases sheet articles cannot be made from prior artground and sieved particles because of excessive drain time or inabilityto control the sheet forming process. The sheet articles formed fromspray-dried particles often have lower flow resistance than sheetarticles made from ground and screened particles and are therefor moreefficient in use.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph of the monodisperse particle-size distribution forspray-dried KNIHEX of Example 3.

FIG. 2 is a graph of the particle-size distribution for ground KNIHEX ofExample 4 (comparative).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In preferred embodiments, sorptive particles useful in the presentinvention include substantially spherical sorptive particles ofhexacyanoferrates, preferably any of potassium cobalt hexacyanoferrate(KCOHEX), potassium zinc hexacyanoferrate (KZNHEX), potassium nickelhexacyanoferrate (KNIHEX) potassium iron hexacyanoferrate (KFEHEX),potassium copper hexacyanoferrate (KCUHEX), potassium cadmiumhexacyanoferrate (KCDHEX), and ammonium iron hexacyanoferrate, (NH₄FEHEX). These particles are useful to sorb cesium (Cs), which may be inits radioactive forms.

It is desirable to have low solubility because it is preferred theadsorbent does not dissolve in use.

It is now recognized that spherical (potassium or ammonium) (metal)hexacyanoferrate particles can be prepared by mixing at 0° to 5° C. anaqueous solution of potassium ferrocyanide (available from AldrichChemical Co., and many other suppliers) with an aqueous solution ofmetal nitrate, buffered to a pH of 5-6 using acetic acid as thebuffering agent. The immediately formed precipitate of (potassium orammonium) (metal) hexacyanoferrate is collected and thoroughly washedwith water. When cobalt nitrate solution or zinc nitrate solution areused, for example, the resulting slurry will contain potassium cobalthexacyanoferrate (KCOHEX) or potassium zinc hexacyanoferrate (KZNHEX)respectively.

The wet precipitate of (alkali metal or ammonium) (metal)hexacyanoferrates which comprises particles of irregular shapes can beslurried with liquid, preferably water, and then spray-dried preferablyusing a spinning disk atomizer and collecting the resultingsubstantially spherical KCOHEX or other metal hexacyanoferrate particleshaving an average size in the range of about 1 to 60 micrometers,preferably 5 to 30 micrometers, most preferably 5 to 15 micrometers,with a most preferred average size of about 9 to 12 micrometers. In thecase of KCOHEX, preferably, the spherical particles are then heateduntil the color changes from green to purplish black but the particlesize remains substantially unchanged. It is desirable to avoid dryingthe hexacyanoferrate particles prior to the spray-drying procedure.

In some applications it may be desirable to limit tile size of the metalor ammonium/metal particles in the slurry that is to be spray-dried. Forexample, particles that are to be incorporated into nonwoven fibrouswebs, matrices, or membranes desirably are spray-dried from slurrieswherein the particles are at most 20 micrometers, preferably at most 10micrometers, and most preferably at most 1 micrometer in average size,to provide the monodisperse, substantially spherical sorbent particlesdescribed above.

It is preferred that the particles in the slurry not be dried prior tosubjecting to the spray drying technique.

Spray-drying of the slurry can be accomplished using well-knowntechniques which include the steps of:

1) atomization, using a spinning disk, of the material introduced intothe dryer;

2) removing of moisture, as, for example, by contact of the materialwith hot gas; and

3) separation of dry product from the exhaust drying agent.

The slurry preferably has a solids content in the range of 3 to 15percent by weight, more preferably 5 to 10 percent by weight, and mostpreferably 5 to 7.5 percent by weight, to ensure smooth operation of theapparatus.

After spray-drying, the particles are free-flowing with most preferredaverage diameters in the range of 9-12 micrometers. When KCOHEX is used,particles are dark green in color. The cyanoferrates preferably areheated at about 115° C. for about 12 hours after drying to achievemaximum absorption capacity for cesium or other metal ions.

The particle can be evaluated for its ion exchange capacity (see Brown,G. N., Carson, K. J., DesChane, J. R., Elovich, R. J., and P. K. Berry.September 1996. Chemical and Radiation Stability of a Proprietary CesiumIon Exchange Material Manufactured from WWI, Membrane and Superlig™644.PNNL-11328, Pacific Northwest National Laboratory, Richland, Wash.) bytesting for the batch distribution coefficient or K_(d) which isdescribed as follows:

The batch distribution coefficient, K_(d) is an equilibrium measure ofthe overall ability of the solid phase ion exchange material to removean ion from solution under the particular experimental conditions thatexist during the contact. The batch K_(d) is an indicator of theselectivity, capacity, and affinity of an ion for the ion exchangematerial in the presence of a complex matrix of competing ions. In mostbatch K_(d) tests, a known quantity of ion exchange material is placedin contact with a known volume of solution containing the particularions of interest. The material is allowed to contact the solution for asufficient time to achieve equilibrium at a constant temperature, afterwhich the solid ion exchange material and liquid supernate are separatedand analyzed. In this application, the batch K_(d) s were determined bycontacting 0.01 g of tile particle with 20 mL of PWTP (Process WasteTreatment Plant simulant solution) (see formulation below).

    ______________________________________                                        PWTP Waste Simulant Composition                                                       Species           Molarity (M)                                        ______________________________________                                        CaCO.sub.3            9.14E.sup.-4                                              Ca(NO.sub.3).sub.2 -4H.sub.2 O 4.27E.sup.-5                                   CaCl.sub.2 1.60E.sup.-5                                                       MgSO.sub.4 2.1E.sup.-4                                                        MgCl.sub.2.6H.sub.2 O 1.18E.sup.-4                                            Ferri-Floc 0.04 ml/L                                                          NaF 4.21 E.sup.-5                                                             Na.sub.3 PO.sub.4.12 H.sub.2 O 2.2 E.sup.-5                                   Na.sub.2 SiO.sub.3.9H.sub.2 O 1.1 E.sup.-3                                    NaHCO.sub.3 1.29 E.sup.-3                                                     K.sub.2 CO.sub.3 1.28 E.sup.-5                                                CsNO.sub.3 3.4 E.sup.-4                                                     ______________________________________                                    

Ferri-Floc™ (Tennessee Chemical Co., Atlanta, Ga.) is a solutioncontaining 10,000 ppm Iron and 25,800 ppm SO₄.

The equation for determining the K_(d) can be simplified by determiningthe concentration of the analyte before and after contact andcalculating the quantity of analyte on the ion exchanger by difference.##EQU1## Where: C_(r) is the initial amount or activity of the ion ofinterest in the feed solution prior to contact,

C_(r) is the amount or activity after contact,

V is the solution volume,

M is the exchanger mass,

F is the mass of dry ion exchanger divided by the mass of wet ionexchanger (F-factor).

K_(d) (normal units are mL/g) represents the theoretical volume ofsolution (mL) that can be processed per mass of exchanger (dry weightbasis) under equilibrium conditions. Lambda, the theoretical number ofbed volumes of solution that can be processed per volume of exchanger,is obtained by multiplying K_(d) by the exchanger bed density, p_(h) (gof resin per mL of resin) as shown below:

λ=K_(d) * P_(h)

The lambda value provides a method for comparing the ion exchangeperformance of a wide variety of materials on a volume basis (e.g., inall ion exchange column).

More preferably, the experimental equipment that was required tocomplete the batch K_(d) determinations included an analytical balance,a constant temperature water bath, an oven for F-factor determinations,a variable speed shaker table, 20-mL scintillation vials, 0.45 μmsyringe filters, the appropriate ion exchanger, and simulant solutions.The particles were all dried thoroughly prior to testing. Approximately0.01 g of each material was contacted with 20 mL of the PWTP solution.The sample bottles were placed into a 25° C. constant temperature bathand shaken lightly for 20 hours. The samples were then filtered with a0.45 micrometer syringe filter to separate the resin material from thesolution and the resulting liquid was analyzed for cesium byICP-MS(Inductively coupled plasma -mass spectrometry) for Cs.

The particles of the invention can be enmeshed in various fibrous,nonwoven webs or matrices which preferably are porous. Types of webs ormatrices include fibrillated polytetrafluoroethylene (PTFE),microfibrous webs, macrofibrous webs, and polymer pulps.

1. Fibrillated PTFE

The PTFE composite sheet material of the invention is prepared byblending the particulate or combination of particulates employed with aPTFE emulsion until a uniform dispersion is obtained and adding a volumeof process lubricant up to approximately one half the volume of theblended particulate. Blending takes place along with sufficient processlubricant to exceed sorptive capacity of tile particles in order togenerate the desired porosity level of the resultant article. Preferredprocess lubricant amounts are in the range of 3 to 200 percent by weightin excess of that required to saturate the particulate, as is disclosedin U.S. Pat. No. 5,071,610, which is incorporated herein by reference.The aqueous PTFE dispersion is then blended with the particulate mixtureto form a mass having a putty-like or dough-like consistency. Thesorptive capacity of the solids of the mixture is noted to have beenexceeded when small amounts of water can no longer be incorporated intothe mass without separation. This condition should be maintainedthroughout the entire mixing operation. Tile putty-like mass is thensubjected to intensive mixing at a temperature and for a time sufficientto cause initial fibrillation of the PTFE particles. Preferably, thetemperature of intensive mixing is up to 90° C., more preferably it isin the range of 0° to 90° C., most preferably 20° to 60° C. Minimizingthe mixing at the specified temperature is essential in obtainingextraction media and chromatographic transport properties.

Mixing times will typically vary from 0.2 to 2 minutes to obtain thenecessary initial fibrillation of the PTFE particles. Initial mixingcauses partial disoriented fibrillation of a substantial portion of thePTFE particles.

Initial fibrillation generally will be noted to be at an optimum within60 seconds after the point when all components have been fullyincorporated into a putty-like (dough-like) consistency. Mixing beyondthis point will produce a composite sheet of inferior extraction mediumand chromatographic properties.

Devices employed for obtaining the necessary intensive mixing arecommercially available intensive mixing devices which are sometimesreferred to as internal mixers, kneading mixers, double-blade batchmixers as well as intensive mixers and twin screw compounding mixers.The most popular mixer of this type is the sigma-blade or sigma-armmixer. Some commercially available mixers of this type are those soldunder the common designations Banbury mixer, Mogul mixer, C. W.Brabender Prep mixer and C. W. Brabender sigma blade mixer. Othersuitable intensive mixing devices may also be used.

The soft putty-like mass is then transferred to a calendering devicewhere the mass is calendered between gaps in calendering rollspreferably maintained at a temperature up to 125° C., preferably in therange of 0° to about 100° C., more preferably in the range of 20° C. to60° C., to cause additional fibrillation of the PTFE particles of themass, and consolidation while maintaining the water level of the mass atleast at a level of near the sorptive capacity of the solids, untilsufficient fibrillation occurs to produce the desired extraction medium.Preferably the calendering rolls are made of a rigid material such assteel. A useful calendering device has a pair of rotatable opposedcalendering rolls each of which may be heated and one of which may beadjusted toward the other to reduce the gap or nip between the two.Typically, the gap is adjusted to a setting of 10 millimeters for theinitial pass of the mass and, as calendering operations progress, thegap is reduced until adequate consolidation occurs. At the end of theinitial calendering operation, the resultant sheet is folded and thenrotated 90° to obtain biaxial fibrillation of the PTFE particles.Smaller rotational angles (e.g., 20° to less than 90°) may be preferredin some extraction and chromatographic applications to reduce calenderbiasing, i.e., unidirectional fibrillation and orientation. Excessivecalendering (generally more than two times) reduces the porosity whichin turn reduces the solvent wicking in thin layer chromatography (TLC)and the flow-through rate in the filtration mode.

During calendering, the lubricant level of the mass is maintained atleast at a level of exceeding the absorptive capacity of the solids byat least 3 percent by weight, until sufficient fibrillation occurs andto produce porosity or void volume of at least 30 percent and preferably40 to 70 percent of total volume. The preferred amount of lubricant isdetermined by measuring the pore size of the article using a CoulterPorometer as described in the Examples below. Increased lubricantresults in increased pore size and increased total pore volume as isdisclosed in U.S. Pat. No. 5,071,610.

The calendered sheet is then dried under conditions which promote rapiddrying yet will not cause damage to the composite sheet or anyconstituent therein. Preferably drying is carried out at a temperaturebelow 200° C. The preferred means of drying is by use of a forced airoven. The preferred drying temperature range is from 20° C. to about 70°C. The most convenient drying method involves suspending the compositesheet at room temperature for at least 24 hours. The time for drying mayvary depending upon the particular composition, some particulatematerials having a tendency to retain water more than others.

The resultant composite sheet preferably has a tensile strength whenmeasured by a suitable tensile testing device such as an Instron(Canton, Mass.) tensile testing device of at least 0.5 MPa. Theresulting composite sheet has uniform porosity and a void volume of atleast 30 percent of total volume.

The PTFE aqueous dispersion employed in producing the PTFE compositesheet of the invention is a milky-white aqueous suspension of minutePTFE particles. Typically, the PTFE aqueous dispersion will containabout 30 percent to about 70 percent by weight solids, the major portionof such solids being PTFE particles having a particle size in the rangeof about 0.05 to about 0.5 micrometers. The commercially available PTFEaqueous dispersion may contain other ingredients, for example,surfactant materials and stabilizers which promote continued suspensionof the PTFE particles.

Such PTFE aqueous dispersions are presently commercially available fromDupont de Nemours Chemical Corp., for example, under the trade namesTeflon™ 30, Teflon™ 30B or Teflon™ 42. Teflon™ 30 and Teflon™ 30Bcontain about 59 percent to about 61 percent solids by weight which arefor the most part 0.05 to 0.5 micrometer PTFE particles and from about5.5 percent to about 6.5 percent by weight (based on weight of PTFEresin) of non-ionic wetting agent, typically octylphenol polyoxyethyleneor nonylphenol polyoxyethylene. Teflon™ 42 contains about 32 to 35percent by weight solids and no wetting agent but has a surface layer oforganic solvent to prevent evaporation. A preferred source of PTFE isFLUON™, available from ICI Americas, Inc. Wilmington, Del. It isgenerally desirable to remove, by organic solvent extraction, anyresidual surfactant or wetting agent after formation of the article.

In other embodiments of the present invention, the fibrous membrane(web) can comprise non-woven, macro- or microfibers preferably selectedfrom the group of fibers consisting of polyamide, polyolefin, polyester,polyurethane, glass fiber, polyvinylhalide, or a combination thereof.The fibers preferably are polymeric. (If a combination of polymers isused, a bicomponent fiber may be obtained.) If polyvinylhalide is used,it preferably comprises fluorine of at most 75 percent (by weight) andmore preferably of at most 65 percent (by weight). Addition of asurfactant to such webs may be desirable to increase the wettability ofthe component fibers.

2. Macrofibers

The web can comprise thermoplastic, melt-extruded, large-diameter fiberswhich have been mechanically-calendered, air-laid, or spunbonded. Thesefibers have average diameters in the general range of 50 μm to 1,000 μm.

Such non-woven webs with large-diameter fibers can be prepared by aspunbond process which is well known in the art. (See, e.g., U.S. Pat.Nos. 3,338,992, 3,509,009, and 3,528,129, the fiber preparationprocesses of which are incorporated herein by reference.) As describedin these references, a post-fiber spinning web-consolidation step (i.e.,calendering) is required to produce a self-supporting web. Spunbondedwebs are commercially available from, for example, AMOCO, Inc.(Naperville, Ill.).

Non-woven webs made from large-diameter staple fibers can also be formedon carding or air-laid machines (such as a Rando-Webber™ Model 12BS madeby Curlator Corp., East Rochester, N.Y.), as is well known in the art.See, e.g., U.S. Pat. Nos. 4,437,271, 4,893,439, 5,030,496, and5,082,720, the processes of which are incorporated herein by reference.

A binder is normally used to produce self-supporting webs prepared bythe air-laying and carding processes and is optional where the spunbondprocess is used. Such binders can take the form of resin systems whichare applied after web formation or of binder fibers which areincorporated into the web during the air laying process.

Examples of common binder fibers include adhesive-only type fibers suchas Kodel™ 43UD (Eastman Chemical Products, Kingsport, Tenn.) andbicomponent fibers, which are available in either side-by-side form(e.g., Chisso ES Fibers, Chisso Corp., Osaka, Japan) or sheath-core form(e.g., Melty™ Fiber Type 4080, Unitika Ltd., Osaka, Japan). Applicationof heat and/or radiation to the web "cures" either type of binder systemand consolidates the web.

Generally speaking, non-woven webs comprising macrofibers haverelatively large voids. Therefore, such webs have low capture efficiencyof small-diameter particulate (reactive supports) which is introducedinto the web. Nevertheless, particulate can be incorporated into thenon-woven webs by at least four means. First, where relatively largeparticulate is to be used, it can be added directly to the web, which isthen calendered to actually enmesh the particulate in the web (much likethe PTFE webs described previously). Second, particulate can beincorporated into the primary binder system (discussed above) which isapplied to the non-woven web. Curing of this binder adhesively attachesthe particulate to the web. Third, a secondary binder system can beintroduced into the web. Once the particulate is added to the web, thesecondary binder is cured (independent of the primary system) toadhesively incorporate the particulate into the web. Fourth, where abinder fiber has been introduced into the web during the air laying orcarding process, such a fiber can be heated above its softeningtemperature. This adhesively captures particulate which is introducedinto the web. Of these methods involving non-PTFE macrofibers, thoseusing a binder system are generally the most effective in capturingparticulate. Adhesive levels which will promote point contact adhesionare preferred.

Once the particulate (reactive supports) has been added, the loaded websare typically further consolidated by, for example, a calenderingprocess. This further enmeshes the particulate within the web structure.

Webs comprising larger diameter fibers (i.e., fibers which averagediameters between 50 μm and 1,000 μm) have relatively high flow ratesbecause they have a relatively large mean void size.

3. Microfibers

When the fibrous web comprises non-woven microfibers, those microfibersprovide thermoplastic, melt-blown polymeric materials having activeparticulate dispersed therein. Preferred polymeric materials includesuch polyolefins as polypropylene and polyethylene, preferably furthercomprising a surfactant, as described in, for example, U.S. Pat. No.4,933,229, the process of which is incorporated herein by reference.Alternatively, surfactant can be applied to a blown microfibrous (BMF)web subsequent to web formation. Polyamide can also be used. Particulatecan be incorporated into BMF webs as described in U.S. Pat. No.3,971,373, the process of which is incorporated herein by reference.

Microfibrous webs of the present invention have average fiber diametersup to 50 μm, preferably from 2 μm to 25 μm, and most preferably from 3μm to 10 μm. Because the void sizes in such webs range from 0.1 μm to 10μm, preferably from 0.5 μm to 5 μm, flow through these webs is not asgreat as is flow through the macrofibrous webs described above.

4. Cast Porous Membranes

Solution-cast porous membranes can be provided by methods known in theart. Such polymeric porous membranes can be, for example, polyolefinincluding polypropylene, polyamide, polyester, polyvinyl chloride, andpolyvinyl acetate fibers.

5. Fibrous Pulps

The present invention also provides a solid phase extraction sheetcomprising a porous fibrous pulp, preferably a polymeric pulp,comprising a plurality of fibers that mechanically entrap activeparticles, and preferably a polymeric hydrocarbon binder, the weightratio of particles to binder being at least 13:1 and the ratio ofaverage uncalendered sheet thickness to effective average particlediameter being at least 125:1.

Generally, the fibers that make up the porous polymeric pulp of the SPEsheet of the present invention can be any pulpable fiber (i.e., anyfiber that can be made into a porous pulp). Preferred fibers are thosethat are stable to radiation and/or to a variety of pHs, especially veryhigh pHs (e.g., pH=14) and very low pHs (e.g., pH=1). Examples includepolyamide fibers and those polyolefin fibers that can be formed into apulp including, but not limited to, polyethylene and polypropylene.Particularly preferred fibers are aromatic polyamide fibers and aramidfibers because of their stability to both radiation and highly causticfluids. Examples of useful aromatic polyamide fibers are those fibers ofthe nylon family. Polyacrylic nitrile, cellulose, and glass can also beused. Combinations of pulps can be used.

Examples of useful aramid fibers are those fibers sold under the tradename Kevlar™ (DuPont, Wil., Del.). Kevlar™ fiber pulps are commerciallyavailable in three grades based on the length of the fibers that make upthe pulp. Regardless or the type of fiber(s) chosen to make up the pulp,tile relative amount of fiber in the resulting SPE sheet (when dried)ranges from about 12.5 percent to about 30 percent (by weight),preferably from about 15 percent to 25 percent (by weight).

Useful binders in the SPE sheet of the present invention are thosematerials that are stable over a range of pHs (especially high pHs) andthat exhibit little or no interaction (i.e., chemical reaction) witheither the fibers of the pulp or the particles entrapped therein.Polymeric hydrocarbon materials, originally in the form of latexes, havebeen found to be especially useful. Common examples of useful bindersinclude, but are not limited to, natural rubbers, neoprene,styrene-butadiene copolymer, acrylate resins, and polyvinyl acetate.Preferred binders include neoprene and styrene-butadiene copolymers.Regardless of the type of binder used, the relative amount of binder inthe resulting SPE sheet (when dried) is about 3 percent to about 7percent, preferably about 5 percent. The preferred amount has been foundto provide sheets with nearly the same physical integrity as sheets thatinclude about 7 percent binder while allowing for as great a particleloading as possible. It may be desirable to add a surfactant to thefibrous pulp, preferably in small amounts up to about 0.25 weightpercent of the composite.

Because the capacity and efficiency of the SPE sheet depends on theamount of particles included therein, high particle loading isdesirable. The relative amount of particles in a given SPE sheet of thepresent invention is preferably at least about 65 percent (by weight),more preferably at least about 70 percent (by weight), and mostpreferably at least about 75 percent (by weight). Additionally, theweight percentage of particles in the resulting SPE sheet is at least 13times greater than the weight percentage of binder, preferably at least14 times greater than the weight percentage of binder, more preferablyat least 15 times greater than the weight percentage of binder.

Regardless of the type or amount of the particles used in the SPE sheetof the present invention, they are mechanically entrapped or entangledin the fibers of the porous pulp. In other words, the particles are notcovalently bonded to the fibers.

Objects and advantages of this invention are further illustrated by thefollowing examples. The particular materials and amounts thereof, aswell as other conditions and details, recited in these examples shouldnot be used to unduly limit this invention.

EXAMPLES Example 1

Potassium cobalt hexacyanoferrate (KCOHEX) was prepared by slowly addingan aqueous solution of 1 part 0.3 M potassium ferrocyamide in an aceticacid buffer (approximately 1.8×10⁻³ M in acetic acid) cooled to between0° C. and 4° C. at 4° C. at pH of between 5 and 6 to an aqueous solutionof 2.4 parts 0.5 M cobalt nitrate (present in excess) cooled to between0° C. and 4° C. with constant stirring. Precipitate was separated fromthe reaction mixture using a centrifuge (Bird Centrifuge Model IIP200,Bird Machine Co., South Walpole, Mass.) by separating the initialprecipitate, then repeatedly slurrying the precipitate in water andre-centrifuging, for a total of four washings. Excess cobalt nitrate ispink in color, so the desired solid material was centrifuged until thesupernatant liquid was no longer pink.

The clean precipitate was again slurried in water at a concentration of7.5 percent by weight, then spray-dried using a Bowen Model BE-1174spray dryer (Niro Atomizer, Inc., Columbia, Md.), as follows:

Inlet air temperature: 210° C.

Atomizing pressure: 276 Kpa (40 psig)

Slurry feed rate: 1.25 gal/hr (4.73 L/hr)

Outlet temperature: 95° C.

Cyclone differential pressure: 4.5 in H₂ O (11.43×10⁻³ kg/cm²).

Recovered dried potassium cobalt hexacyanoferrate represented 92 percentyield, and was observed under an Olympus BH2 microscope (OlympusAmerica, Inc., Melville, N.Y.) to be spherical in shape. Measurement ofthe particles using a Horiba Model LA-900 Particle Size Analyzer (HoribaInstruments, Inc., Irvine, Calif.), showed an average particle size ofapproximately 11 micrometers and a particle size distribution of fromabout 7 micrometers to about 60 micrometers.

The spray-dried material was dark green in color. The sphericalparticles were heated in an air-vented oven at approximately 100° C.until they changed to a purplish-black color. Recovery from heating wasessentially quantitative, and no loss of size or shape was noted as aresult of the heating procedure.

Example 2 (Comparative)

Potassium cobalt hexacyanoferrate was prepared as described in Example1, except that the solid precipitate obtained from the final centrifugewash was spread out in a thin layer about 2.5 cm thick on a drying trayand heated in air at 110° C.-115° C. until the solid had dried and itscolor had changed from green to purplish-black. The dried solid wassubjected to cryogenic hammer-milling by means of a Model D Comminutor(Fitzpatrick Co., Elmhurst, Ill.) using a 325-mesh screen (0.044 mmsieve opening) and the resulting powder was sized in a hydrocyclone(Richard Mozley Ltd., Cornwall, UK). Particles in the range of fromabout 1 to about 50 micrometers in their largest dimension wereretained, with an average particle size of approximately 10-15micrometers. Yield of sized particles was approximately 60 percent. Asexpected from a milling process, particles were typically irregular inshape (significant deviation from spherical) and a broad particle sizedistribution was obtained. The yield was significantly less in theparticle size range desired.

Example 3

Potassium zinc hexacyanoferrate (KZNHEX) was prepared by slowly addingan aqueous solution of 1 part 0.5 M potassium ferrocyanide (0.28 moles)(pH was adjusted to pH 5.0-5.1 using concentrated acetic acid) cooled tobetween 0° C. and 4° C. at to an aqueous solution of 2.0 parts 0.3 Mzinc nitrate (0.34 moles) cooled to between 0° C. and 4° C. withconstant stirring. Precipitate was separated from the reaction mixtureusing a centrifuge (Centrifuge Model 460G, International EquipmentCompany, Boston, Mass.) by separating the initial precipitate, thenrepeatedly slurrying the precipitate in water and re-centrifuging, for atotal of four washings.

The clean precipitate was again slurried in water at a concentration of7.5 percent by weight, then spray-dried using a Niro Atomizer, Model 68Order # 093-1413-00, Serial #2402 spray dryer (Niro Atomizer, Inc.,Columbia, Md.), as follows:

Inlet air temperature: 190° C.

Spinning Disc RPM's: 400 KPa (58 psig)

Slurry feed rate: 2.4 L/hr

Outlet temperature: 85° C.

Cyclone magnahelic pressure: 0.47 in H₂ O (1.19×10⁻³ kg/cm²).

Recovered dried potassium zinc hexacyanoferrate was observed under anSEM (Cambridge model S240, LEO Electromicroscopy, Inc., Thornwood, N.Y.)to be spherical in shape. Measurement of the particles using a HoribaModel LA-900 Particle Size Analyzer (Horiba Instruments, Inc., Irvine,Calif.), showed an average particle size (average diameter) ofapproximately 11.8 micrometers with about 20 weight percent less than 5μm in size.

The raspberry-shaped particles were heated in an air-vented oven atapproximately 115° C. for 18 hours. Recovery from heating wasessentially quantitative, and no loss of size or shape was noted as aresult of the heating procedure.

Example 4 (Comparative)

Potassium zinc hexacyanoferrate was prepared as described in Example 3,except that the solid precipitate obtained from the final centrifugewash was spread out in a thin layer about 2.5 cm thick on a drying trayand heated in air at 115° C. for 18 hours until the solid had dried. Thedried solid was broken up from its clumps using a mortar and pestle. Theaverage particle size of the material was 0.636 micrometers so nofurther grinding was done. The particles were observed under an SEM(Cambridge Model S240), and were observed to be typically irregular inshape (significant deviation from spherical). All particles were lessthan 1 μm in size.

Example 5

Potassium nickel hexacyanoferrate (KNIHEX) was prepared by slowly addingan aqueous solution of 1 part 0.3 M potassium ferrocyanide (0.86 moles)(pH was adjusted to pH 5.0-5.1 using concentrated acetic acid) cooled tobetween 0° C. and 4° C. at to an aqueous solution of 1.44 parts 0.3 Mnickel nitrate (1.23 moles) cooled to between 0° C. and 4° C. withconstant stirring. Precipitate was separated from the reaction mixtureusing a centrifuge (Centrifuge Model 460G International EquipmentCompany, Boston, Mass.) by separating the initial precipitate, thenrepeatedly slurrying the precipitate in water and re-centrifuging, for atotal of four washings.

The clean precipitate was again slurried in water at a concentration of7.5 percent by weight, then spray-dried using a Niro Atomizer Model 68,order #093-1413-00, Serial #2402 spray dryer (Niro Atomizer, Inc.,Columbia, Md.), as follows:

Inlet air temperature: 192° C.

Spinning Disc RPM's: 400 KPa (58 psig)

Slurry feed rate: 2.4 L/hr

Outlet temperature: 79.3° C.

Cyclone magnahelic pressure: 0.47 in H₂ O (1.19×10⁻³ kg/cm²).

Recovered dried potassium nickel hexacyanoferrate was observed under anSEM (Cambridge model S240) to be spherical in shape. Measurement of theparticles using a Horiba Model LA-900 Particle Size Analyzer (HoribaInstruments, Inc., Irvine, Calif.), showed an average particle size ofapproximately 9.5 micrometers with about 10 weight percent of theparticle less than 5 micrometers.

The spherical particles were heated in an air-vented oven atapproximately 115° C. for 18 hours. Recovery from heating wasessentially quantitative, and no loss of size or shape was noted as aresult of the heating procedure.

FIG. 1 shows that the particle size distribution of the KNIHEX particleswas monodisperse.

Example 6 (Comparative)

Potassium nickel hexacyanoferrate was prepared as described in Example5, except that the solid precipitate obtained from the final centrifugewash was spread out in a thin layer about 2.5 cm thick on a drying trayand heated in air at 115° C. for 18 hours until the solid had dried. Thedried solid was broken up from its clumps using a mortar and pestle. Thematerial was then ground with a ball mill using Zirconia media for 90minutes. The average particle size was 16.77 micrometers with about 30%of the material less than 5 micrometers in size. The particles wereobserved under an SEM (Cambridge Model S240) and found to be typicallyirregular in shape (significant deviation from spherical).

FIG. 2 shows the particle size distribution of the ground KNIHEXparticles was large compared to that of Example 5 (FIG. 1) where theparticle size distribution was narrow.

Example 7

Ammonium ferric hexacyanoferrate (NH₄ Fe^(III) [Fe^(II) (CN)₆ ] wasprepared by slowly adding an aqueous solution of 1 part 0.3 M ammoniumferrocyanide (0.86 moles) (pH was adjusted to pH 5.0-5.1 usingconcentrated acetic acid) cooled to between 0° C. and 4° C. at to anaqueous solution of 1 parts 0.5 M ferric nitrate (0.86 moles) cooled tobetween 0° C. and 4° C. with constant stirring. Precipitate wasseparated from the reaction mixture using a centrifuge (Centrifuge Model460G, International Equipment Company, Boston, Mass.) by separating theinitial precipitate, then repeatedly slurrying the precipitate in waterand re-centrifuging, for a total of four washings.

The clean precipitate was again slurried in water at a concentration of7.5 percent by weight, then spray-dried using a Niro Atomizer, Serial#2402 spray dryer (Niro Atomizer, Inc., Columbia, Md.), as follows:

Inlet air temperature: 190° C.

Spinning Disc RPM's: 400 KPa (58 psig)

Slurry feed rate: 2.4 L/hr

Outlet temperature: 67.6° C.

Cyclone magnahelic pressure: 0.47 in H₂ O (1.19×10⁻³ kg/cm²)

Recovered dried ammonium ferric hexacyanoferrate was observed under anSEM (Cambridge model S240) to be spherical in shape. Measurement of theparticles using a Horiba Model LA-900 Particle Size Analyzer (HoribaInstruments, Inc., Irvine, Calif.), showed an average particle size ofapproximately 16.11 micrometers with less than about 10 weight percentof the particles less than 5 micrometers.

The spherical particles were heated in an air-vented oven atapproximately 115° C. for 18 hours. Recovery from heating wasessentially quantitative, and no loss of size or shape was noted as aresult of the heating procedure.

Example 8 (Comparative)

Ammonium ferric hexacyanoferrate was prepared as described in Example 7,except that the solid precipitate obtained from the final centrifugewash was spread out in a thin layer about 2.5 cm thick on a drying trayand heated in air at 115° C. for 18 hours until the solid had dried. Thedried solid was broken up from its clumps using a mortar and pestle. Thematerial was then ground with a ball mill using Zirconia media for 40minutes. The average particle size was 5.00 micrometers with about ⁹⁸ %of the material less than 5 micrometers in size. The particles weretypically irregular in shape (significant deviation from spherical).

Example 9

Potassium nickel hexacyanoferrate (KNIHEX) was prepared by slowly addingan aqueous solution of 1 part 0.3 M potassium ferrocyanide (0.29 moles)(pH was adjusted to 5.0-5.1 using concentrated acetic acid) cooled tobetween 0° C. and 4° C. at to an aqueous solution of 1.44 parts 0.3 Mnickel nitrate (0.42 moles) cooled to between 0° C. and 4° C. withconstant stirring. Precipitate was separated from the reaction mixtureusing a centrifuge (Centrifuge Model 460G, International EquipmentCompany, Boston, Mass.) by separating the initial precipitate, thenrepeatedly slurrying the precipitate in water and re-centrifuging, for atotal of four washings.

The clean precipitate was again slurried in water at a concentration of7.5 percent by weight, then spray-dried using a Niro Atomizer, Serial#2402 spray dryer (Niro Atomizer, Inc., Columbia, Md.), as follows:

Inlet air temperature: 192° C.

Spinning Disc RPM's: 400 KPa (58 psig)

Slurry feed rate: 2.4 L/hr

Outlet temperature: 79.3° C.

Cyclone magnahelic pressure: 0.47 in H₂ O (1.19×10⁻³ kg/cm²).

Recovered dried potassium nickel hexacyanoferrate was observed under anSEM (Cambridge model S240) to be spherical in shape. Measurement of theparticles using a Horiba Model LA-900 Particle Size Analyzer (HoribaInstruments, Inc., Irvine, Calif.), showed an average particle size ofapproximately 12 micrometers with less than about 10% of the particleless than 5 micrometers.

The spherical particles were heated in an air-vented oven atapproximately 115° C. for 18 hours. The yield was 78% by weight of thereacted prepared material. No change in size or shape was noted as aresult of the heating procedure.

Example 10 (Comparative)

Potassium nickel hexacyanoferrate was prepared as described in Example9, except that the solid precipitate obtained from the final centrifugewash was spread out in a thin layer about 2.5 cm thick on a drying trayand heated in air at 115° C. for 18 hours until the solid had dried. Thedried solid was broken up from its clumps using a mortar and pestle. Thematerial was then ground in a freezer mill (SPEX model 6700 SPEXIndustries, Inc., Edison, N.J.) for 6 minutes. The material was sievedwith a 200 mesh screen (74 μm) to remove large particles and thenhydrocycloned to remove fines below 5 μm. The average particle sizeafter hydrocycloning was 21 micrometers with about 27 weight percent ofthe material less than 5 micrometers in size. The particles weretypically irregular in shape (significant deviation from spherical). Thetotal yield after grinding, sieving and hydrocycloning was 3.7%.

Example 11

Ammonium ferric hexacyanoferrate was prepared by slowly adding anaqueous solution of 1 part 0.3 M ammonium ferrocyanide (0.35 moles) (pHwas adjusted to pH 5.0-5.1 using concentrated acetic acid) cooled tobetween 0° C. and 4° C. at to an aqueous solution of 2 parts 0.5 Mferric nitrate (0.50 moles) cooled to between 0° C. and 4° C. withconstant stirring. Precipitate was separated from the reaction mixtureusing a centrifuge (Centrifuge Model 460G, International EquipmentCompany, Boston, Mass.) by separating the initial precipitate, thenrepeatedly slurrying the precipitate in water and re-centrifuging, for atotal of four washings.

The clean precipitate was again slurried in water at a concentration of7.5 percent by weight, then spray-dried using a Niro Atomizer, Serial#2402 spray dryer (Niro Atomizer, Inc., Columbia, Md.), as follows:

Inlet air temperature: 192° C.

Spinning Disc RPM's: 400 KPa (58 psig)

Slurry feed rate: 2.4 L/hr

Outlet temperature: 79.3° C.

Cyclone magnahelic pressure: 0.47 in H₂ O (1.19×10⁻³ kg/cm²).

Recovered dried ammonium ferric hexacyanoferrate represented 75 percentyield, and was observed under an SEM (Cambridge model S240) to bespherical in shape. Measurement of the particles using a Horiba ModelLA-900 Particle Size Analyzer (Horiba Instruments, Inc., Irvine,Calif.), showed all average particle size or approximately 14.0micrometers with less than about 10% of the particle less than 5micrometers.

The spherical particles were heated in an air-vented oven atapproximately 115° C. for 18 hours. No change of size or shape was notedas a result of the heating procedure.

Example 12 (Comparative)

Ammonium ferric hexacyanoferrate was prepared as described in Example11, except that the solid precipitate obtained from the final centrifugewash was spread out in a thin layer about 2.5 cm thick on a drying trayand heated in air at 115° C. for 18 hours until the solid had dried. Thedried solid was broken up from its clumps using a mortar and pestle. Theaverage particle size was 1.15 μm micrometers with 98.5% of the materialless than 5 micrometers. The particles were typically irregular in shape(significant deviation from spherical) and were too small to processfurther. 98.5% of the material was less than 5 μm in size. The yield was1.5%.

Example 13

A particle-filled porous web was prepared from the spherical potassiumcobalt hexacyanoferrate particles of Example 1. An agitated slurry of 20g Kevlar™ 11:306 dry aramid fiber pulp (DuPont, Wilmington, Del.) in2000 g water was blended in a 4 L Waring™ blender at a low speed for 30seconds, then mixed with 0.25 g Tamol 850™ dispersant (Rohm & Haas Co.,Philadelphia, Pa.), followed by 8.75 g (3.5 g dry weight) Goodrite™1800X73 styrene-butadiene latex binder aqueous slurry (B. F. GoodrichCo., Brecksville, Ohio). Blending was continued for 30 seconds at a lowspeed. To this mixture was added 53.6 g potassium cobalthexacyanoferrate, followed by 20 g powdered alum (aluminum sulfate), andstirring was continued for an additional minute. The mixture was pouredinto a Williams sheet mold (Williams Apparatus Co., Watertown, N.Y.)equipped with a 930.3 cm² porous screen having pores of approximately0.14 mm (100 mesh) at the bottom to allow water to drain. The resultingwet sheet was pressed in a pneumatic press (Mead Fluid Dynamics,Chicago, Ill.) at approximately 620 KPa for approximately five minutesto remove additional water. Drain time was 25 seconds, and no loss ofKCOHEX could be observed in the drain water. Finally, the porous web wasdried in an oven at 250° F. (121° C.) for 120 minutes.

Example 14 (Comparative)

Example 13 was repeated using ground and sized particulate as obtainedin Example 2 in place of spray-dried particles. Drain time was 60seconds. This showed that prior art particles loaded in a web, requiredmore than 2 times the drain time compared to spray-dried particles ofthe invention loaded in the web.

Example 15

A 90 mm diameter disk having a thickness of 3.2 mm was die cut from theparticle-filled porous web of Example 13. The disk was placed in astainless steel disk holder (Cole Parmer Instrument Co., Niles, Ill.)fitted with inlet and outlet pipes, so that the effective disk diameterwas 80 mm. The disk was washed with water, then an aqueous solutioncontaining 12 mg/L cesium ion was pumped first through twocanister-style prefilters in series (Filtrete™ Cartridges, MemteeAmerica Corp., Timonium, Md.) then through the disk at a flow rate of105 mL/min., which corresponds to 6.49 bed volumes/min., wherein bedvolume means the volume of the disk. Cesium removal, reported asC/C_(o), where C represents the cesium concentration in the effluent andC_(o) represents initial cesium concentration, as a function of bedvolumes is shown in Table 1. The pressure drop across the membrane wasfound to be constant at 55 KPa throughout the evaluation.

                  TABLE 1                                                         ______________________________________                                               Bed Volumes                                                                            C/Co                                                          ______________________________________                                                 0      0                                                                389 0                                                                         778 0                                                                        1167 0.019                                                                    1557 0.167                                                                    1946 0.358                                                                    2335 0.60                                                                     2724 0.717                                                                    3113 0.792                                                                    3506 0.833                                                                    4186 0.917                                                                  ______________________________________                                    

The data of Table 1 show that spherical KCOHEX particles in a porous webare very effective in removing Cesium ions from aqueous solution.

Example 16

A particle-filled porous web was prepared from the spherical potassiumzinc hexacyanoferrate particles of Example 3. Into a 4 L Waring™ blenderwas added 2000 g water with 0.25 g Tamol 850™ dispersant (Rohm & HaasCo., Philadelphia, Pa.) and then 9.6 g Kevlar™ 1F306 dry aramid fiberpulp (DuPont, Wilmington, Del.) was added and blended at low speed for30 seconds. To this slurry was then added 36 g of the particle fromExample 3 with blending on low speed, followed by 6.15 g (2.4 g dryweight) Goodrite™ 1800X73 styrene-butadiene latex binder aqueous slurry(B. F. Goodrich Co., Brecksville, Ohio). Blending was continued for 30seconds at a low speed. To this mixture was added 20 g of a 25% solutionof Alum (aluminum sulfate in water), and stirring was continued for anadditional minute. After which 1 gram of a 1% solution of Nalco 7530(Nalco Chemical Company, Chicago, Ill.) was added and the solution mixed3 seconds on low. The mixture was poured into a Williams sheet mold(Williams Apparatus Co., Watertown, N.Y.) equipped with a 413 cm² porousscreen having a pore size of 80 mesh (approximately 177 μm) at thebottom to allow water to drain, drain time was 120 seconds. Theresulting wet sheet was pressed in a pneumatic press (Mead FluidDynamics, Chicago, Ill.) at approximately 551 KPa for approximately fiveminutes to remove additional water. Finally, the porous web was dried ona handsheet dryer for 120 minutes at 150° C.

Example 17

Example 16 was repeated using ground and sized particulate as obtainedin Example 4 in place of spray-dried particles. Drain time was 240seconds. This showed that prior art particles loaded in a web required 2times the drain time compared to spray-dried particles of the inventionloaded in a web.

Example 18

A particle-filled porous web was prepared from the spherical potassiumnickel hexacyanoferrate particles of Example 5. Into a 4 L Waring™blender was added 2000 g water with 0.25 g Tamol 850™ dispersant (Rohm &Haas Co., Philadelphia, Pa.) and then 14.35 g Kevlar™ 1F306 (12 g ,83.5% solids) aramid fiber pulp (DuPont, Wilmington, Del.) was added andblended at low speed for 30 seconds. To this slurry was then added 45 gof the particle from Example 5 with blending on low speed, followed by7.69 g (3.0 g dry weight) Goodrite™ 1800× X 73 styrene-butadiene latexbinder aqueous slurry (B. F. Goodrich Co., Brecksville, Ohio). Blendingwas continued for 30 seconds at a low speed. To this mixture was added20 g of a 25% solution of Alum (aluminum sulfate in water), and stirringwas continued for an additional minute. After which 1 gram of a 1%solution of Nalco 7530 (Nalco Chemical Company, Chicago, Ill.) was addedand the solution mixed 3 seconds on low. The mixture was poured into aWilliams sheet mold (Williams Apparatus Co., Watertown, N.Y.) equippedwith a 413 cm² porous screen having a pore size of 80 mesh(approximately 177 μm) at the bottom to allow water to drain, drain timewas 30 seconds. The resulting wet sheet was pressed in a pneumatic press(Mead Fluid Dynanics, Chicago, Ill.) at approximately 551 KPa forapproximately five minutes to remove additional water. Finally, theporous web was dried on a handsheet dryer for 120 minutes at 150° C.

Example 19

Example 18 was repeated using ground particles obtained in Example 6 inplace of spray-dried particles. Drain time was 45 seconds. This showedthat prior art particles loaded in a web required more thin 1.5 timesthe drain time compared to spray-dried particles of the invention loadedin a web.

Example 20

A particle-filled porous web was prepared from the spherical ammoniumferric hexacynoferrate particles of Example 7. Into a 4 L Waring™blender was added 2000 g water with 0.25 g Tamol 850™ dispersant (Rohm &Haas Co., Philadelphia, Pa.) and then 12.9 g Kevlar™ 1F306 (10.8 g,83.5% solids) aramid fiber pulp (DuPont, Wilmington, Del.) was added andblended at low speed for 30 seconds. To this slurry was then added 40.5g of the particle from Example 7 with blending on low speed, followed by6.9 g (2.7 g dry weight) Goodrite™ 1800 X 73 styrene-butadiene latexbinder aqueous slurry (B. F. Goodrich Co., Brecksville, Ohio). Blendingwas continued for 30 seconds at a low speed. To this mixture was added20 g of a 25% solution of Alum (aluminum sulfate in water), and stirringwas continued for an additional minute. After which 1 gram of a 1%solution of Nalco 7530 (Nalco Chemical Company, Chicago, Ill.) was addedand the solution mixed 3 seconds oil low. The mixture was poured into aWilliams sheet mold (Williams Apparatus Co., Watertown, N.Y.) equippedwith a 413 cm² porous screen having a pore size of 80 mesh(approximately 177 μm) at the bottom to allow water to drain, drain timewas 15 seconds. The resulting wet sheet was pressed in a pneumatic press(Mead Fluid Dynamics, Chicago, Ill.) at approximately 551 KPa forapproximately five minutes to remove additional water. Finally, theporous web was dried on a handsheet dryer for 120 minutes at 150° C.

Example 21

Example 20 was repeated using ground and sized particulate as obtainedin Example 8 place of spray-dried particles. Drain time was 120 seconds.This showed that the prior particles loaded in a web required 8 timesthe drain time compared to spray-dried particles of the invention loadedin a web.

Example 22

A 25 mm diameter disk having a thickness of 1.98 mm was die cut from theparticle-filled porous web of Example 20, spray dried NH₄ Fe^(III)(Fe^(II) (CN)₆). The disk weight was 0.69 g with 72.7% particle, weightof particle in membrane at 22 mm (flow area) was 0.39 g. The disk wasplaced in a stainless steel disk holder (Cole Parmer Instrument Co.,Niles, Ill.) fitted with inlet and outlet pipes, so that the effectivedisk diameter was 22 mm. The disk was washed with water, then an aqueoussolution containing 50 mg/L cesium ion was pumped through the disk at aflow rate of 5 mL/min.,, which corresponds to 6.6 bed volumes/min.,wherein bed volume means the volume of the disk. Cesium removal,reported as C/C_(o), where C represents the cesium concentration in theeffluent and C_(o) represents initial cesium concentration, as afunction of bed volumes is shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Bed Volumes      C/Co   .increment.P (Kpa)                                    ______________________________________                                         0               0      21                                                       32 0.016 21                                                                   66 0.188 21                                                                   99 0.447 21                                                                  132 0.498 21                                                                  232 0.597 21                                                                  298 0.662 21                                                                  365 0.731 21                                                                  464 0.707 21                                                                  597 0.738 21                                                                  730 0.826 21                                                                  1129  0.866 27                                                              ______________________________________                                    

The data of Table 2 show that spherical NH₄ FEHEX particles in a porousweb are effective in removing cesium ions from aqueous solution. Theback pressure build up was constant (i.e. about 21 KPa) during about 3/4of the run, and increased slightly to about 27 KPa at the end of therun.

Example 23 (Comparative)

A 25 mm diameter disk having a thickness of 2.16 mm was die cut from theparticle-filled porous web of Example 21, ground NH₄ Fe^(III) (Fe^(II)(CN)₆). The disk weight was 0.76 g and contained 72% by weight particle,therefore at 22 mm (flow area) the particle weight was 0.34 g. The diskwas placed in a stainless steel disk holder (Cole Parmer Instrument Co.,Niles, Ill.) fitted with inlet and outlet pipes, so that the effectivedisk diameter was 22 mm. The disk was washed with water, then an aqueoussolution containing 50 mg/L cesium ion was pumped through the disk at aflow rate of mL/min., which corresponds to bed volumes/min., wherein bedvolume means the volume of the disk. Cesium removal, reported asC/C_(o), where C represents the cesium concentration in the effluent andC_(o) represents initial cesium concentration, as a function of bedvolumes is shown in Table 3. The pressure drop across the membraneincreased to 151 KPa during the evaluation.

                  TABLE 3                                                         ______________________________________                                        Bed Volumes      C/Co   .increment.P (Kpa)                                    ______________________________________                                         47              0.34   41                                                       94 0.54 41                                                                   141 0.50 48                                                                   188 0.45 55                                                                   235 0.43 55                                                                   376 0.37 66                                                                   471 0.34 69                                                                   565 0.37 76                                                                   753 0.40 90                                                                   942 0.45 97                                                                   1130  0.52 110                                                              ______________________________________                                    

The data of Table 3 show that irregular NH₄ FEHEX particles in a porousweb are effective in removing cesium ions from aqueous solution. Theback pressure build up during the evaluation was 1.51 KPa which was morethan five times greater than back-pressure for the spray-dried materialin Example 22.

Example 24

Comparative capacity data derived from K_(d) determination is presentedin Table 4 below, both for spray-dried and ground material.

                  TABLE 4                                                         ______________________________________                                                 Example     Processing                                                                              Capacity (g Cs/g                                 Particle Number Method particle)                                            ______________________________________                                        K.sub.2 ZnFe(CN).sub.6 *                                                               4           Raw product                                                                             0.1125                                           K.sub.2 ZnFe(CN).sub.6 3 Spray dried 0.115                                    KNiFe(CN).sub.6 * 6 Ground 0.0997                                             KNiFe(CN).sub.6 5 Spray dried 0.1093                                        ______________________________________                                         *comparative                                                             

The data of Table 4 show that the capacities for spray-dried materialsof the invention were improved compared to a raw product of groundparticles.

Example 25 Mixed Metal Hexacyanoferrate adsorbents

Table 5, below, gives compositions and capacities for cesium for anumber of adsorbents with a composition of potassium metalhexacyanoferrate where metal may be cobalt, copper, nickel, zinc ormixtures of those metals.

                  TABLE 5                                                         ______________________________________                                                       Metal/metal                                                                             Capacity                                               Particle ratio mMol/gram                                                    ______________________________________                                        KCOCUHEX       Co/Cu = 1 0.84                                                   KCOCUHEX Co/Cu = 1/3 0.84                                                     KCOCUHEX Co/Cu = 3 0.79                                                       KCOCIHEX Co/Ni = 1 0.81                                                       KCOCIHEX Co/Ni = 1/3 0.78                                                     KCOHEX KCOHEX 0.85                                                            KCOZNHEX Co/Zn = 1 0.79                                                       KCOZNHEX Co/Zn = 1/3 0.81                                                     KCOZNHEX Co/Zn = 3 0.83                                                       KCOZNHEX Cu/Zn = 1 0.79                                                       KZNHEX KZNHEX 0.85                                                            KCUHEX KCUHEX 0.76                                                            KCOCUHEX Co/Cu = 1/2 0.77                                                     KCOCUHEX Co/Cu = 2 0.81                                                       KCOZNHEX Co/Zn = 1/2 0.41                                                   ______________________________________                                    

All of these adsorbents were prepared essentially following theprocedure given in Example 1 using appropriate starting materials. Inall cases an excess of metal (as compared with potassium) existed andamounts of the solutions of each metal were adjusted to yield the ratiosshown in the second column.

All of these compositions had good or high capacity for cesium and allhad reasonably low solubility.

Various modifications and alterations that do not depart from the scopeand intent of this invention will become apparent to those skilled inthe art. This invention is not to be unduly limited to the illustrativeembodiments set forth herein.

I claim:
 1. A composite sheet comprising spray-dried, monodisperse,substantially spherical, sorbent (alkali metal or ammonium) (metal)hexacyanoferrate particles and combinations thereof enmeshed in anonwoven fibrous web or a cast membrane wherein said metal is selectedfrom the group consisting of Periodic Table (CAS version) Groups VIII,IB, and IIB, said spray-dried sorbent particles having an average sizein the range of 1 to 500 micrometers, and said spray-dried sorbentparticles being active towards Cs metal ions.
 2. The composite sheetaccording to claim 1, wherein said particles are selected from the groupconsisting of potassium and ammonium (metal) hexacyanoferrates whereinmetal is selected from the group consisting of Fe, Ru, Os, Rh, Ir, Co,Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and Hg.
 3. The composite sheet accordingto claim 1 wherein said particles are selected from the group consistingof potassium cobalt hexacyanoferrate (KCOHEX), potassium zinchexacyanoferrate (KZNHEX), potassium copper hexacyanoferrate, potassiumnickel hexacyanoferrate, potassium cadmium hexacyanoferrate, potassiumiron hexacyanoferrate, and ammonium iron hexacyanoferrate.
 4. Thecomposite sheet according to claim 3 wherein said particles comprisepotassium cobalt hexacyanoferrate.
 5. The composite sheet according toclaim 3 wherein said particles comprise zinc hexacyanoferrate.
 6. Thecomposite sheet according to claim 3 wherein said particles comprisepotassium copper hexacyanoferrate.
 7. The composite sheet according toclaim 3 wherein said particles comprise potassium nickelhexacyanoferrate.
 8. The composite sheet according to claim 3 whereinsaid particles comprise potassium cadmium hexacyanoferrate.
 9. Thecomposite sheet according to claim 3 wherein said particles comprisepotassium iron hexacyanoferrate.
 10. The composite sheet according toclaim 3 wherein said particles comprise ammonium iron hexacyanoferrate.11. The composite sheet according to claim 3 wherein said particlescomprise a combination of any of said hexacyanoferrates.
 12. Thecomposite sheet according to claim 1 wherein said particles have anaverage size in the range of 1 to 60 micrometers.
 13. The compositesheet according to claim 1 wherein said nonwoven fibrous web comprisesfibrillated polytetrafluoroethylene (PTFE).
 14. The composite sheetaccording to claim 1 wherein said nonwoven fibrous web comprises afibrous pulp.
 15. The composite sheet according to claim 1 wherein saidnonwoven fibrous web comprises a macroporous fibrous web.
 16. Thecomposite sheet according to claim 1 wherein said nonwoven fibrous webcomprises a microporous fibrous web.
 17. The composite sheet accordingto claim 1 further comprising a hydrocarbon binder.
 18. The compositesheet according to claim 1 wherein said particles have an average sizein the range of 9 to 12 micrometers.